A well-known feature of mobile telephones is so called caller-identification, or caller-ID, whereby the caller's telephone number is displayed on the user interface (UI) of a telephone receiving a call. In order to do this, the receiving telephone decodes a signal, which is associated with the call, containing the caller's number. If the caller's details are stored in the memory of the receiving phone, the UI may also display the caller's name. Caller-ID enables the user of a telephone receiving a call to check as to who is calling them, before they decide to answer.
The UI of a mobile telephone also commonly displays prompts to events stored in the telephone's memory, such as events entered in the telephone's calendar.
The disadvantage of a telephone's UI displaying such information is that the information can be read, not only by the telephone's owner, but also by any other person in the telephone's near vicinity. Hence the telephone's owner is not provided with the privacy he or she may desire. In attempting to protect their privacy, the owner may be constrained into not using some of the features offered by their telephone, such as alerts to private events recorded in the calendar. Alternatively, the user may decide to leave the telephone hidden from view, thus potentially missing calls and other types of alert altogether.